The present invention relates to acoustic wave devices.
Monolithic electronic devices in which information is stored on a slowly propagating surface or bulk acoustic wave have found considerable use in communications, radar and other signal processing systems. Among materials which can be used to advantage in fabricating such devices are crystalline materials in which regions, or domains, interfacing at a so-called domain wall, can coexist stably. For most directions of propagation through the crystal, acoustic waves propagate at different velocities in adjacent domains. In addition, a domain wall can be made to move predictably by applying appropriate electrical control signals to the crystalline body. The crystalline material itself may be, for example, a rare earth molybdate such as terbium molybdate or gadolinium molybdate. In these materials, which are ferroelectric, the application of an electrical control signal along the axis of spontaneous electrical polarization (polar axis) in a region of the material which includes a domain wall effects an interchange of the two nonpolar axes immediately adjacent one side of the domain wall or the other, depending on the control signal polarity. This, by definition, effects the domain wall movement.
The above-described crystallographic properties have been exploited in the prior art to provide a variable delay line for acoustic waves. Input and output transducers for either surface acoustic waves (SAW) or bulk acoustic waves (BAW) are so disposed on the crystalline body that the output transducer receives acoustic waves which have propagated within at least two domains. The delay between the input and output transducers is thus varied by moving the domain wall so as to vary the relative propagational path lengths within the two domains. The control signal is applied to the crystalline body by way of a pair of electrodes disposed on opposite faces thereof, each electrode comprising a homogeneous, low impedance plate which covers substantially the entire face on which it is disposed.
The prior art further teaches that the acoustic device may include a second domain wall between the transducers in order to compensate for acoustic wave refraction at the first domain wall. Since each electrode covers substantially the entire face on which it is disposed, the applied control signal causes both domains to move either towards or away from each other. This can be disadvantageous since the precise position of each wall cannot be accurately controlled. Moreover, special care must be taken to ensure that the walls do not meet each other, in which case the entire body would be transformed into a single domain. It thus may be desirable to have the position of one of the domain walls fixed. To this end, the prior art suggests that a crystal defect be provided at the desired stationary-wall position. There are several drawbacks to this approach, however. For example, it is difficult to provide a crystal defect which is assured to reliably hold the domain wall in place. In addition, fabrication of devices with substantially identical defects to ensure device uniformity in mass production would also be difficult. Moreover, a crystal defect can serve as a nucleation site for further, unwanted domain walls.